1. Field of the Invention
The present invention relates generally to a relatively rotating sliding element. More particularly, the invention relates to a sliding element which reduces a friction coefficient on its sliding face and provides an effective seal for a process fluid on the sliding face.
2. Description of the Related Art
Related art of the present invention on a silicon carbide sintered element is found in U.S. Pat. No. 5,080,378. The description and drawings disclose a mechanical seal whose cross sectional view is shown in FIG. 8.                The mechanical seal is used in pumps, refrigerators or the like.        
In FIG. 8, a mechanical seal 100 is located between a rotary shaft 130 and a housing 140. The mechanical seal 100 is used for sealing a fluid in pumps, refrigerators or the like.
In this mechanical seal 100, a seal ring 101 made of sintered porous silicon carbide is fitted over the rotary shaft 130. The rotary seal ring 101 retains a seal face 102 on its side surface. Furthermore, packings 120A, 120B are disposed in a step shoulder 103 of the inner diameter surface of the rotary seal ring 101 to seal against the rotary shaft 130.                The packings 120A, 120B are pressed by a gland ring 105 and seal the interface of the rotary shaft 130 and the rotary ring 101. A support ring 109 which is fixed to the rotary shaft 130 by means of a socket screw 108 supports a spring element 106, and the gland ring 105 is resiliently urged by the spring element 106.        
An opposing seal face 111 which forms a slidably sealing contact with the seal face 102 is disposed in a fixed ring 110. The fixed ring 110 is secured via O-rings 115,115 to a bore of the housing 140 through which the rotary shaft 130 extends.                This fixed ring 110 is made of carbon.        In a conventional mechanical seal 100 arranged as mentioned above, the seal ring 101 and the fixed seal ring 110 slide with respect to each other while maintaining a sealing therebetween in order to seal a higher pressure side P1 from a lower pressure side P2.        The seal ring 101 has a sintered silicon carbide body in which spherical pores whose average diameter is in a range of from 0.010 mm to 0.040 mm are spread within its crystalline structure and a lubricant captured inside the pores improves its sliding resistance.        
The pores located in the sliding face of the sintered silicon carbide body are fabricated by adding polystyrene beads in a pre-sintering process and then resolving and sublimating them in a temporary sintering. This process provides a sintered silicon carbide body with a plurality of pores isolating from each other and scattered inside the crystalline structure of crystal grains. From a fabrication standpoint, a difficulty in high compression molding causes a decrease in dimensional accuracy of the molded product.                Also polystyrene beads resolved in the sintering process decreases strength of a sintered material as a sliding element. These pores are simply arranged at random on the sliding face and a mere preservation of lubricant in the pores does not yield decrease of the friction coefficient of the sliding face as expected.        
There is an enhanced version of mechanical seal shown in FIG. 9 which improves the aforementioned problems in terms of the strength decrease and the process fluid leakage of a sliding element.                This mechanical seal has the same constitution as what is shown in FIG. 8. The sliding face of a fixed ring, not shown in the figure, retained in the housing forms a sealing contact with the sliding face of a driven ring securely attached to the rotary shaft as shown in FIG. 9 for sealing a process fluid.        The sliding face 155A of the driven ring 155 retains a lot of concaves 156. Minimum width of the concave 156 is in a range of from 30×10−6 m to 100×10−6 m while the maximum width is in a range of from 60×10−6 m to 500×100−6 m. Moreover, the maximum width is more than twice in dimension of the minimum width. The concaves 156 are oriented at random relative to a rotational direction of the sliding face 155A.        
The concaves 156 serve a purpose of preserving a process fluid which enters into between the sliding face 155A of the driven ring 155 and the sliding face of the fixed ring. That is, the process fluid entering from the outer circumference side of the driven ring is trapped and stored in the concave 156 before the fluid reaches the inner circumference edge. The fluid stored in the concave 156 is pushed toward the circumferentially backward direction of the concave 156 due to viscosity of the fluid and rotary motion of the driven ring, and a portion of the fluid exceeding the storage capacity of the concave starts to leak from the outer circumferential edge of the concave 156, which moves between the relatively sliding faces and is eventually trapped by an adjacent concave 156.                Thus, the process fluid is pushed toward the outer circumferential edge and to the backward direction relative to the rotary motion of the sliding faces.        
Dimension of the concave 156 is so small not only that it can not provide enough force to retain the process fluid on the sliding face but also that it does not induce enough pumping effect to push back the process fluid to where it comes from. Therefore, neither decrease of the friction coefficient of the sliding face of the driven ring 156 nor decrease of the heat generation due to sliding friction can be expected. Also it is difficult to reduce a sliding resistance in case of a slow rotational speed of the driven ring.
Also there has been other prior art sliding element for mechanical seals. The sliding face of the sliding element for mechanical seals disposes a plurality of dimples lined up like a groove along a longitudinal direction which is vertical to a sliding direction.                Dynamic pressure generated within the dimples is fairly large. Thus, a lubricant oil film of the fluid formed is so thin that the lubricant oil preserved in the dimples does not provide a long-term reduction effect of the friction coefficient of the sliding faces.        
The present invention is introduced to resolve the above mentioned problems. A primary technical goal which this invention tries to achieve is to form a film of process fluid on a sliding face for reducing a frictional resistance and enhancing a seal performance. Another goal is to prevent heat generation on the sliding face. Yet another goal is to prevent a wear of the sliding face for a long-term assurance of the performance of the sliding faces.                Yet another goal is not only to prevent a decrease of a seal performance of the sliding element at a slow rotational speed but also to reduce a frictional resistance at such a slow speed.        